Fungus is opportunistic pathogens in humans. Fungus typically does not infect healthy tissues, yet once tissue defense mechanisms have been compromised, they can readily infect the tissue. One typical model of this opportunistic fungal infection is candidiasis, which is caused by Candida albicans. 
Candida albicans occurs as normal flora in the oral cavity, genitalia, large intestine, and skin of approximately 20% of humans. The risk of infection increases in children and pregnant women; people who use certain antibiotics or have nutritional and organic disease or immunodeficiency (e.g., AIDS) or trauma; and people with invasive devices, e.g., pacemakers. Candida albicans and its close relatives account for nearly 80% of nosocomial fungal infections and 30% of deaths from nosocomial infections in general.
Historically, opportunistic, fungal infections in hospitalized patients were rather unusual. Textbooks from the past described these agents as common contaminants with weak pathogenic potential, and infections were considered extreme deviation from the normal. Older ideas concerning these so-called harmless contaminants are now challenged because in those days immunodeficient and debilitated patients had died from their afflictions long before fungal infection took place. However, currently, with the advent of innovative surgeries, drugs, and other therapies that maintain such patients for expected periods, the survival rates of patients have significantly increased and the number of compromised patients has thus increased. One clinical dilemma that cannot be completely eliminated, even with rigorous disinfections, is the exposure of such patients to potential fungal pathogens from even normal flora. Fungal infections in such high-risk patients progress rapidly and are difficult to diagnose and treat. In one study, fungi caused approximately 40% of the deaths from clinically acquired infections. Up to 5% of all nosocomial opportunistic fungi cause infections.
Fungi also present special problems in chemotherapy. A majority of chemotherapeutic drugs used in treating bacterial infection are generally ineffective in combating fungal infection. Moreover, the similarity between fungal and human cells often means drug toxic to fungal cells are capable of harming human cells. A few drugs with special antifungal properties have been developed for treatment of systemic and superficial fungal infections. For example, macrolide polyenes represented by amphotericin B, have a structure that mimics some cell membrane lipids. Amphotericin B which is isolated from a species of streptomycin is by far the most versatile and effective of all antifungal drugs. The azoles are broad-spectrum antifungal drugs with a complex ringed structure. As one of the most effective azole drugs, fluconazole, is used in patients with AIDS-related mycoses.
Magnaporthe grisea is the pathogen of a devastating fungal disease of rice plants known as rice blast. The fungus can also cause a similar disease in over 50 grasses, including economically important crops such as barley, wheat, and millet. Fusarium is another important genus of fungal pathogens, responsible for devastating diseases such as cereal scab.